1. Field of the Invention
The present invention relates to optical discs and methods for manufacturing the same. In particular, the present invention relates to a high-strength, durable optical disc whose abrasion resistance and scratch resistance at the light-incident surface side, where writing is performed, are increased to prevent the deterioration of the writing and reading characteristics, and to a method for manufacturing the same.
2. Description of the Related Art
The packing density and storage capacity of optical discs for recording various types of information, such as audio data, image data, and computer data, have been remarkably increasing, and new types of products, such as compact discs (CD media), mini Discs (MD media), and digital versatile discs (DVD media), are being introduced on the market one after another, accordingly. In general, such an optical disc includes a recording layer and a reflective layer on a substrate, and is further covered with a protective layer. The substrate, from which writing and reading laser beams come in, is generally formed of polycarbonate (PC).
As the optical disc becomes widespread, writing or reading errors increase due to scratches at the light-incident side surface of the substrate. Accordingly, it becomes required to provide a hard coating layer over the light-incident side surface of the substrate. More specifically, since PC constituting the substrate does not have an adequate abrasion resistance, it has been proposed that a hard coating layer is provided over the surface of the PC substrate to increase the abrasion resistance. This technique has been put into practice for some optical discs.
In order to increase the abrasion resistance or scratch resistance of the hard coating layer, Japanese Unexamined Patent Application Publication Nos. 2001-139884 and 2002-30150 have disclosed hard coating agents respectively containing a hydrolysate containing silicon of polyfunctional organoalkoxysilane and a specific dimethylsiloxane compound, and optical discs using the respective hard coating agents. Also, Japanese Unexamined Patent Application Publication No. 2001-344821 has disclosed an optical disc including a hard coating layer comprising at least two transparent cured material sub-layers whose outermost layer and adjoining internal layer contain a specific material; Japanese Unexamined Patent Application Publication No. 2001-344816 has disclosed an optical disc including a hard coating layer comprising a transparent cured film containing a specific polyfunctional material and polysilazane.
However, as the demand for optical discs grows, optical discs having superior abrasion resistance and scratch resistance become desired and the foregoing techniques have become unsatisfactory. In particular, fine cracks causing recording errors can occur at the surface of the hard coating layer during storage. A new technique is desired to prevent the occurrence of the cracks.